1. Field of the Invention
The present invention relates to a variable-focal-length lens system and an imaging apparatus. More specifically, the present invention relates to a variable-focal-length lens system used for a digital video camera, a digital still camera, or the like and having a zoom ratio of more than five times, and an imaging apparatus using the variable-focal-length lens system.
2. Description of the Related Art
In the related art, to perform recording in digital still cameras, a method exists in which the quantity of light of a subject's image formed on the surface of an imaging device using a photoelectric converter such as a CCD (Charge Coupled Device) sensor or a CMOS (Complementary Metal Oxide Semiconductor) sensor is converted into an electric output by the imaging device, thereby recording the subject's image.
Recent advances in microprocessing techniques have led to increases in the processing speed of central processing units (CPUs) and in the level of integration of recording media, making it increasingly possible to handle high-speed processing of large volumes of image data not possible in the past.
Higher levels of integration and miniaturization have been achieved for light-receiving devices as well. The higher levels of integration are making recording of higher spatial frequencies possible, and the miniaturization of light-receiving devices has led to the overall miniaturization of cameras.
However, the above-mentioned increases in the level of integration and miniaturization lead to narrower light-receiving areas of individual photoelectric converters, and the effect of noise becomes significant due to the resulting decrease in electric output. Attempts that have been made to prevent this problem include increasing the quantity of light reaching photoelectric converters by increasing the aperture ratio of the optical system, and placing minute lens elements (so-called microlens array) in front of individual photoelectric converters.
However, in exchange for guiding light beams falling in between adjacent photoelectric converters onto the photoelectric converters, this microlens array places constraints on the position of the exit pupil of the lens system (the distance from the image plane to the exit pupil).
This is because if the exit pupil position of a lens system becomes closer to the photoelectric converters, that is, if the angle formed between the principal ray reaching each photoelectric converter and the optical axis becomes larger, off-axis light beams going toward the periphery of the picture plane form large angles to the optical axis, and the off-axis light beams do not reach the photoelectric converters, resulting in insufficient light quantity.
As digital still cameras have become more and more common in recent years, users' needs are becoming increasingly diversified. In particular, there are a growing number of digital still cameras equipped with zoom lenses with high zoom ratios, and the number of zoom lenses with zoom ratios of more than five times is particularly increasing.
Generally speaking, these zoom lenses with high zoom ratios use a positive, negative, positive, positive four-group type as a zoom type representing the configuration of the zoom lenses. This positive, negative, positive, positive four-group type zoom lens includes a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power, which are arranged in this order from the object side.
In the case of this positive, negative, positive, positive four-group type zoom lens, as the lens position state changes from the wide-angle end state with the shortest focal length to the telephoto end state with the longest focal length, the individual lens groups from the first to third lens groups move in such a way that the space between the first lens group and the second lens groups increases and the space between the second lens group and the third lens group decreases, and variations in the image plane position (focus position) of the imaging device are compensated for by the movement of the four lens group that is driven separately (for example, Japanese Unexamined Patent Application Publication No. 2008-146016).
As a zoom lens used as an interchangeable lens or the like, a positive, negative, positive, negative, positive five-group type exists as a zoom type that realizes high zoom ratio or high optical performance by increasing the number of movable lens groups.
This five-group type zoom lens includes a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, a fourth lens group having negative refractive power, and a fifth lens group having positive refractive power, which are arranged in order from the object side.
In the case of such a five-group type zoom lens, as the lens position state changes from the wide-angle end state to the telephoto end state, the individual lens groups move in such a way that the space between the first lens group and the second lens groups increases, the space between the second lens group and the third lens group decreases, the space between the third lens group and the fourth lens groups increases, and the space between the fourth lens group and the fifth lens group decreases (for example, Japanese Unexamined Patent Application Publication No. 2007-108398).
There are also five-group type zoom lenses applied to non-interchangeable type cameras with no limitations on back focus (for example, Japanese Unexamined Patent Application Publication No. 2007-264174 and Japanese Unexamined Patent Application Publication No. 2067-264395).